The LWF Blog

Fire Engineering Design and Risk Assessment – Hazard Classifications – Part 15

October 5, 2017 1:17 pm

In LWF’s Fire Engineering Design and Risk Assessment blog series for Architects and others involved in building design, we have been looking recently at sprinkler protection and the hazard classifications which exist to indicate the type of risk with which the sprinkler must cope. In Part 14, we discussed Light Hazard risks and began to talk about Ordinary Hazard risks, Part 15 will continue from that point.


While it is not always possible to wholly position a building and its associated business with a risk level based on the nature of its business, it is often helpful. In the Ordinary Hazard classification it is most common to find hotels, restaurants and cafes, and industrial buildings, each of which are likely to include commercial standard kitchens.


While the majority of the kitchen area would be responsive to sprinkler activation if there were a fire, deep fat fryers require more careful handling. Water droplets from a sprinkler system (or any other source) entering the pool of hot oil are extremely hazardous because of the incredible rate of expansion when water becomes steam.  The resulting eruption is like a small explosion which would cause the extremely hot oil to splatter over quite a large area and might result injuries to any building occupants in the area.


It is possible to get sprinklers which operate at higher pressures, meaning that water droplets cannot fall into the fryer and cause this type of reaction, but more commonly, industrial kitchens are often equipped with dry powders or foam for this purpose.


The High Hazard classification encompasses commercial and industrial occupancies which contain abnormal fire loads. This might be due to the processes which take place as part of production, the nature of the goods in storage or the height to which goods are stored.


The risks are subdivided into four categories


 High Hazard Process Risks

 High Piled Storage Risks

 Potable Spirit Storage

 Oil and Flammable Liquid Hazards


Because the building is subject to one or more of the named conditions, the fire growth curve is likely to be fast to ultra-fast. It has the ability to grow to be a very high output fire with a fuel load of more than 1000MJ.m-2 . In cases such as this, it is unlikely that the fire service would be able to control the fire easily.


Standard sprinkler configurations are also likely to find fire suppression impossible and it is for this reason that High Hazard occupancies are subject to some alterations to the sprinkler provision to accommodate the additional risks.


The sprinkler head spacing should be reduced, discharge densities are increased and water supply durations must be increased.


In Part 16 we will continue to talk about High Hazard classification buildings and the potential fire safety solutions. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.


Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.



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